The chancellor of the exchequer endeavours to present a sober and serious image as a man who can steer us through crisis. But it seems that George Osborne was not always so buttoned up.

Details have emerged of Osborne's wild university days as a member of the notorious Bullingdon Club. The all-male dining club, to which prime minister David Cameron also belonged as an undergraduate, is open only to sons of aristocratic families or the super-rich and is famed for its riotous behaviour. A 1992 photograph of Osborne in tie and tails with his fellow members, including multimillionaire financier Nat Rothschild, has been much reproduced.

Osborne, who belonged to the Bullingdon while studying modern history at Magdalen College, Oxford, in the early 1990s, has never spoken in detail of what he got up to as a member, preferring to draw a veil over his youthful antics. But in an interview with the Observer Magazine, one of Osborne's Bullingdon contemporaries has spoken for the first time about some of the astonishing escapades to which the future chancellor bore witness. They include an alcohol-fuelled party that degenerated into a fist fight, allegations of cocaine use by another member of the club, and an evening during which the members trashed a Michelin-starred restaurant.

The contemporary, who asked not to be identified, said that on one evening in 1992, shortly after the famous photograph of Osborne was taken, the Bullingdon members boarded a double-decker bus to Waddesdon Manor in Buckinghamshire, the Rothschild family seat.

"It started to get really out of control," he said. "I remember a guest comatose on the lawn, being tended to by a butler who was applying cold towels to his forehead, trying to bring him round. One of the guys got into a fist fight because he was Italian and a football match was on and there'd been some racial taunting. Plates had been thrown. As usual, it escalated.

"I think George was mildly alarmed. He was enjoying the food and wine, enjoying watching the football, and I just remember him looking at me with raised eyebrows at what was going on. I never saw him take drugs."

On a different occasion, with Osborne also present, the source recalled one Bullingdon member trying to snort lines of coke on an open-top bus. "I said to him: 'You're stupid, it's blowing away', and his response was: 'I can afford it'."

On another occasion, Osborne and the Bullingdon went for a meal at a Michelin-starred restaurant in Berkshire where, coincidentally, the comedian Lenny Henry and his then wife, Dawn French, were having dinner. The source said: "A couple of the boys started getting obnoxious and talking about their family wealth and Lenny Henry said: 'Actually, sod off.' There was a slight altercation when a member put a cigar out on someone else's lapel and it turned into a fist fight and furniture was broken. It was horrible, horrible. We used to smash everything up then pay a cheque saying: 'It's OK, we can pay for it'."

Unlike many of his cabinet colleagues (including William Hague, a fellow Magdalen alumnus), there has never been any sense until now that Osborne was particularly involved in student politics. But the Observer can reveal that, as a 19-year-old, he did stand for the post of entertainments representative in his college junior common room. In fact, his electioneering was so enthusiastic that his rival for the position wrote a letter of complaint to the JCR vice-president outlining the future Conservative MP's underhand tactics. Dated 15 November 1990, the letter accuses Osborne of "electorate malpractice" on several counts including "the dissemination of five different wordings of posters, instead of the mandatory two" and "the attempt on the part of Mr George Osborne to pervert the democratic process by electioneering in the JCR".

It was written by Rupert Harding, who won the election. Harding, who now runs a language school in Finland, has little memory of the event. Contacted by the Observer, he said: "Perverting the democratic process I think meant going up to people after Neighbours and asking them to vote for him."

In October 2008, it was claimed that Osborne had tried to solicit a £50,000 donation from the Russian aluminium magnate Oleg Deripaska while on Deripaska's yacht in Corfu along with Labour peer Lord Mandelson. Such a move would have been a violation of the law against political donations by foreign citizens.

A formal complaint was made to the Electoral Commission, which rejected the claims.

• This article was amended on 13 January 2012 to remove a name.

• This correction was published on 15 January 2012:Our article on the chancellor, George Osborne ("The player", Observer Magazine) and a related story ("George Osborne and the Bullingdon club: what the chancellor saw", News), referred to a number of incidents involving Nathaniel Rothschild at an event at the Rothschild family home. Mr Rothschild confirms that he denies that these incidents took place. We are happy to make his position clear.